Lucky for you! This is a year late but there's a person named Lucina who is basically just like him! Long story short, time travel blah blah, lucina pretends to be marth in the past etc, but it's like the same :P

Yes, Marth is a guy. I always thought Marth was a girl until my younger brother (Age 10) told me...Brother: "NU! MARFH IZ A GUWY."Me: "How so?"Brother: "Wiztin twoo hiz dieuhwawg!"Me(listening to part of Marth's dialog): O.oMe: *Thinks that bro edited the dialog to make him sound like a boy, so looks up online: 'Is Marth a boy or girl from Fire Emblem?'*Computer Screen: Marth is the prince of Altea, and the main character in the game Fire Emblem.

So there you have it. Sorry my comment is so long, I wanted to give as much info as I could to prove my answer. I am a video game geek. :3